Prevention of Biofouling in Marine Environments
Continued exposure to seawater can lead to detrimental biofouling and corrosion in marine industrial components, particularly hydraulic piston rods.
Surface engineering is extensively employed to provide hydraulic rods with improved corrosion and tribological properties on surfaces that interact with the seals mounted in the hydraulic cylinders. Degradation of the sealing surface of the rod by corrosion, wear, or any other process that affects the surface finish, will result in seal damage, leading to a loss of sealing performance, and ultimately causing system failure. Electrolytic hard chrome (EHC) is the dominant surface engineering technology used in most traditional hydraulic applications, but is generally not suitable for use in severe service applications due to EHC’s poor corrosion resistance. Furthermore, EHC has fallen out of favour in more traditional applications due to its negative environmental and OHS impact. There has therefore been significant research into developing hard chrome alternative technologies (HCAT) for hydraulic service.
High velocity oxy-fuel (HVOF) thermal spray coatings, especially tungsten carbide – cobalt chrome (WC-CoCr), has found favour in many commercial aerospace applications. Laser cladding of steel or stainless steel hydraulic rods with corrosion resistant nickel-based alloys has shown good performance in corrosive hydraulic applications, and has found favour in many marine applications. There are certain environments where these HCATs have proven unsuitable, notably immersed stagnant marine applications, where the corrosion environment is severe and there is the added risk of degradation associated with biofouling. Biofouling is the growth of marine organisms on surfaces immersed in the sea, and these organisms can be very tenaciously adherent and abrasive. During cycling of the hydraulic rod, these organisms can become trapped between rod and cylinder and degrade the sealing surface though surface residue or pull-out, resulting in damage to the seal.
A suitable HCAT should provide adequate corrosion resistance, and also have the ability to operate in a biofouling environment – either by having some inherent biofouling resistance, or by having suitable damage tolerance to resist aggressive removal of biofouling without unacceptable degradation of the seal contact surface. Furthermore, the HCAT must be able to achieve and maintain a hydraulic seal compatible surface finish.
Swinburne University, in collaboration with UST, DMTC and McTaggert Scott Engineering conducted an investigation into the development of an HCAT for protecting hydraulic rods in an immersed marine environment subject to biofouling. Several coating systems were developed, focussing on materials selection and spray parameter optimisation. These were trialled in various marine environments along the Australian coast. Two nickel based WC cermets, applied using optimised HVOF parameters and processed to the appropriate surface finish were shown to exhibit excellent marine corrosion and biofouling resistance, and through field testing have proven to be viable candidates for use in immersed marine service subject to biofouling.